My Book needs are power supply as well as connection to computer. The newer ones can connect through USB, Firewire or eSATA.

My Passport are a lot smaller (seriously, the size of a passport, just a little thicker, and you can get different colours). Only connect through USB (which is still fairly fast, btw, I can transfer around 30GB at once in under 10 minutes), and doesn't need a power supply just straight into the computer.

I was going to get a My Book but carrying around a power supply was a huge turn off, especially since at Uni it'd be hard to find a power point near the computer.

Plus, I'd gather most notebooks don't have eSATA or firewire ports.